Indexable milling insert

ABSTRACT

The invention relates to an indexable milling insert ( 3 ) having a polygonal basic shape, comprising four peripheral sides, which meet each other in pairs in one the one hand two acute corners ( 15 ), and on the other hand two obtuse corners ( 16 ). Between chip surfaces ( 17 ) on the top side of the insert and flank surfaces ( 18 ) along the peripheral sides, cutting edges ( 19 ) are formed, which individually comprise on the hand a major edge portion ( 20 ), and, on the other hand, a rounded minor edge portion ( 21 ), formed in connection with an acute corner ( 15 ). Characteristic of the invention is the combination that the individual major edge portion ( 20 ) is slightly arched and that at least one connecting surface ( 4 ) is formed in the bottom side of the insert, of the type that comprises a plurality of parallel ridges spaced-apart by grooves having a cross-section-wise tapering shape and which are intended to co-operate with an analogous connecting surface adjacent to a chip channel in a cutting-insert-carrying milling head.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates to an indexable milling insert having apolygonal basic shape and a generally positive cutting geometry,including a top side, a bottom side and four peripheral sides, a firstpair of mutually opposite sides which meet a second pair of mutuallyopposite sides in two acute angled corners as well as two obtuse angledcorners, and also two cutting edges for cutting material, formed betweenchip surfaces on the top side of the insert and flank surfaces along theperipheral sides, which edges individually comprise on the one hand amajor edge portion, and, on the other hand, a rounded, minor edgeportion, formed in connection with an acute angled corner, which minoredge portion extends in an extension of the major edge portion, whereinthe major edge portion is at least partially arched in shape.

GENERAL PRIOR ART

[0002] Inserts of the above generally mentioned kind are used in millingtools of the router type, usually in order to create cells or cavitiesin workpieces, which are delimited by comparatively thin walls. Millingtools of this kind are particularly and frequently used in order tomachine aluminium with the purpose of providing light frameworkcomponents of the type that is used in the construction of aircraft. Inthese contexts, many different requirements are made of the millingtools ad the inserts thereof, the most important ones of which are ahigh capacity of chip removing as well as a suitable surface structureof the walls which define the cells.

[0003] Previously known inserts for router cutters are formed with planebottom sides and plane supporting side surfaces, whereby the inserts aremounted in the appurtenant insert seats or seating by the fact that thebottom side of the individual insert is pressed against a plane bottomin the seat at the same time as one or more supporting side surfaces arepressed against supporting side walls in the seat. Inserts of this typemay give, in the best case, an acceptable surface smoothness as long asthey work at moderate cutting speed by running the cutters at moderaterotational speeds. Within the technical field in question, however, thedevelopment is being intensively driven in the direction of ever fastercutting operations. Thus, in the milling of aluminium, machining mayalready today take place at a cutting or peripheral speed in the orderof 10 000 m/min or more, whereby the rotational speed of the millingcutter may exceed 25 000 rpm. Under such conditions, it has proveddifficult to attain suitable surfaces on the walls that delimit themiller cells. These difficulties are based on a tendency of the insertsto be dislodged from the positions thereof; something that in turnresults in that the active edge, more precisely the major edge portionthereof, cannot retain the intended, theoretical position thereof. Inother words, the individual insert will move in an uncontrolled way inrelation to the appurtenant milling head. The resulting irregularitiesin the surface machined by the major edge portion of the insert may bemanifested differently. In some cases, plane stripes or ribbon-like partsurfaces, which extend at an angle to each other are created (observedin a cross-section, the surface becomes zigzag-shaped or in otherregards irregular). In other cases, the irregularities may be shaped ascross-section-wise convexly curved, long narrow part surfaces, spacedpert by thin, inwardly directed flute formations. Said flute formationsmay give rise to cracks in the wall material.

[0004] Aims and Features of the Invention

[0005] The present invention aims at obviating the above-mentioneddisadvantages and at providing an improved milling insert of the typeinitially mentioned. Thus, a primary aim of the invention is to providean insert by means of which the shape of the material surface machinedby the major edge portion of the insert may be determined in acontrolled and exact way. An additional aim is to provide a millinginsert which, in co-operation with the appurtenant milling head, iscapable of achieving thin walls and so-called floors in connection witha milled cell or cavity, while maintaining fast milling operations andfine production tolerances. Thus, the insert should be capable ofreliably retaining the geometry and exact position thereof even when themilling head is run at high a rotational speed, e.g. 25 000 r/min ormore. Yet another aim of the invention is to provide a milling insertcapable of entering and leaving the material in the work piece in adynamically stable and controlled way, and which can be used forso-called ramping. Another aim of the invention is to provide an insertcapable of milling material with low energy consumption, i.e. with highefficiency. Furthermore, the insert should be able to efficientlymachine material having a comparatively low modulus of elasticity, e.g.such materials as aluminium or the like.

[0006] According to the invention, at least the primary aim is attainedby the features that are given in the characterizing section of claim 1.Preferred embodiments of the invention are furthermore defined in thedependent claims.

[0007] The invention is based on a unique combination of features, eachindividually previously known within the field of chip removingmachining, viz. on the one hand a cutting edge, the major edge portionof which is arched, and, on the other hand, a connecting surface ofserration type formed on the bottom side of the insert. Due to the factthat the major edge portion (which executes the major part of the chipremoving work) of the cutting edge is arched, the insert itself, as wellas the individual insert seat in the milling head, may be given a markedpositive geometry. At the same time the connecting surface which isprovided with ridges, which is formed on the bottom side of theinsert—in co-operation with a corresponding connecting surface on themilling head—ensures a rigid and exact fixing of the insert in thedesired position. This means that the two end points between which thearched major edge portion of the cutting edge extend, may be located in,and for a long duration retain, well-defined, radial positions.

[0008] Further Elucidation of Prior Art

[0009] From U.S. Pat. No. 6,102,630, a milling insert 2 previouslyknown, which on the bottom side thereof has ridged connecting surfaceintended to co-operate with a similar connecting surface on a tool. Inthis case, however, the major edge portion of the insert is straight.

[0010] From U.S. Pat. No. 4,940,369 is previously known, an insert ofthe router-type with a major cutting edge extending lengthways and whichis slightly arched. In this case the insert however lacks any form ofridged connecting surface, and as a result the clamping of the cuttinginsert in the corresponding insert seat will be unstable andunsatisfactory. As a consequence of this it will not be possible todictate the surface of the machined material in a controlled and exactmanner when ensuring fine machining tolerances during high speed millingoperations.

BRIEF DESCRIPTION OF THE APPENDED DRAWINGS

[0011] In the drawings:

[0012]FIG. 1 is a perspective view of a milling tool comprising amilling head and an insert included therein according to the invention,

[0013]FIG. 2 is an enlarged, partly cut end view showing the insert fromone end of the milling head,

[0014]FIG. 3 is a side view of the milling tool shown in connection witha milled cell in a workpiece,

[0015]FIG. 4 is a perspective view showing the insert according to theinvention as seen obliquely from above and from a short end,

[0016]FIG. 5 is a perspective view showing the same insert as seen froma long side,

[0017]FIG. 6 is a planar view from below of the same insert,

[0018]FIG. 7 is a side view of the insert,

[0019]FIG. 8 is a planar view from above of The same insert,

[0020]FIG. 9 is a cross-section through the insert shown together with afixing screw,

[0021]FIG. 10 is an end view of the insert,

[0022]FIG. 11 is a schematic planar view in which t major edge portionscomprised in the insert are illustrated with an exaggerated arc-shape,and

[0023]FIG. 12 is a cross-section in a very enlarged scale showing apreferred design of the ridged connecting surface of the insert.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0024]FIG. 1 shows a milling tool in the form of a head 1 having arotationally symmetrical basic shape, in the envelope surface of whichtwo chip channels 2 are formed. In one of said chip channels, an insert3 according to the invention is mounted. In FIG. 2 it may be seen howthe insert 3 on a bottom side has at least one connecting surface 4 ofthe type that comprises a plurality of parallel ridges, spaced-apart viagrooves and having a cross-section-wise tapering shape. Said connectingsurface is intended to co-operate with a similar, ridged connectingsurface 5 which forms a insert seat or seating in the chip channel 2.

[0025] In FIG. 3, the milling tool is shown in connection with aworkpiece in which a cavity or cell 6 has been milled, which isdelimited by sidewalls 7 and a so-called floor 8. The internal surfaceof the sidewalls 7 is designated 9, while the upper surface of the floor8 is designated 10.

[0026] It should be pointed out already now, that the geometry of thetool is markedly positive inasmuch as the insert 3 in the mounted statehas considerable axial and radial angles. The same are determined by theangles of he insert seat or the connecting surface 5 in relation to therotation axis of the milling head. In preferred embodiments, the axialangle of the contact surface 5 is within the range of 10-15°, while theradial angle may be within the range of 10-25°.

[0027] Reference is now made to FIGS. 4-12, which in detail illustratethe design of the insert according to the invention. The insert has apolygonal basic shape and a topside generally designated 11, a bottomside 12, as well as four peripheral sides 13, 14. Of said peripheralsides, a pair of first, opposite sides 13 meet the second pair ofmutually opposite sides 14 in on the one hand two acute corners 15, and,on the other hand, two obtuse corners 16. Cutting edges, generallydesignated 19, are formed between chip surfaces 17 of the topside of theinsert and flank surfaces 18 along the peripheral sides. The same arebelow denominated primary cutting edges. Each individual primary cuttingedge 19 includes a major edge portion 20, as well as a minor edgeportion 21, having a markedly round, e.g. partially circular shape,formed in connection with an acute corner 15. The major edge portion 20extends from a first or front end point 22 to a rear end point 23. Therounded minor edge portion 21 extends in the extension of the major edgeportion 20, more precisely from the point 22 to a point 24.

[0028] In the shown, preferred embodiment, the insert has a rhomboidalbasic shape, whereby the two major edge portions 20 extend along longsides 13 of the insert. In FIGS. 6 and 8, the insert is shown insertedin a conceived co-ordinate system with a first co-ordinate axis orX-axis, and a second co-ordinate axis or Y-axis. The shape of the insertis symmetrical in relation to said two co-ordinate axes.

[0029] In FIG. 6 it may be seen that the insert in the preferredembodiment has two connecting surfaces 4, which are placed on both sidesof a central, through hole 25 in the insert. Each one of said twoconnecting surfaces 4 comprises a plurality of ridges, spaced-apart bygrooves, which are mutually parallel (the connecting surfaces 4 aredescribed closer below in connection with FIG. 12). Said ridges andgrooves are parallel to the X-axis. The two connecting surfaces arespaced-apart from the hole, i.e. end at a distance therefrom.

[0030] The individual major edge portion 20 extends generally obliquelyin relation to the X-axis so far that the front end point 22 is situatedat a greater distance from the X-axis than the rear end point 23. Theangle of inclination a (see FIG. 8) between the individual major edgeportion 20 and the X-axis should be within the range of 0.5-3°, suitably0.5-2°. In the example, the angle α amounts to 1.8°.

[0031] In accordance with a characteristic feature of the invention, themajor edge portion 20 of the cutting edge 19 is arched. This arc-shapeis obtained by the fact that the part of the flank surface 18 that isconnected to the major edge portion 20 is made with a slightly markedconvex curvature. The curvature, however, is so small that the arc-shapeof the major edge portion 20 is not visible to the naked eye in FIGS.4-10. Therefore, the major edge portion is shown schematically with anexaggeratedly large arc-shape in FIG. 11. Ideally, the arc line whichforms the major edge portion 20 has an elliptical basic shape, whereby achord K of the ellipse arc extends between the aforementioned end points22, 23. According to the invention, the arc height should be within therange of 0.05-1% of the length L_(k) of the chord K. In a concreteembodiment example, the chord has a length of 20 mm, whereby the archeight B amounts to 0.02 nm. (=0.1%).

[0032] In FIG. 7, a neutral plane N is shown with a dash-dotted line,which is parallel to the bottom side of the insert as the same isrepresented by the peaks or apex surfaces of the ridges which areincluded in the connecting surfaces 4.

[0033] Reference is now made to FIGS. 4 and 5, in which it may be seenthat the cutting edges 19 together with the chip surfaces 17 are formedon border-like, peripheral material portions in relation to which avalley-like surface 26 is countersunk. Said valley surface is plane andparallel to the neutral plane N. Centrally on the valley surface, anelevated material portion 27 is formed. In the example, where the insertis formed with a central hole 25, the elevated material portion 27 is inthe form of an endless collar in which a head 28 to a fixing screw 29(see FIG. 9) may be housed in a countersunk and protected state. Thus,as is seen in FIG. 9, the topside of the screw head 28 is situated on alower level than a surrounding plane top surface 30 of the collar 27when the screw is tightened. From the plane topside 30 of the collar, aside surface 31 extends obliquely downwards/outwards towards the valleysurface 26. Said side surface, which is endless, may in practise serveas a chip breaker for the chips that are separated by the individual,active cutting edge 19. In the shown embodiment, in which the insert hasa rhomboid basic shape, the collar 27 has an elongated, oval shape andextends diagonally, essentially in the direction between the obtusecorners 16 of the insert (see also FIG. 8).

[0034] As is further seen in FIGS. 4 and 5, a individual flank surface18 extends downwards from the cutting edge 19 to a shoulder designated32, which delimits a narrowed plinth surface 33 under the flank surfaceitself. Said plinth surface 33 may advantageously be of a larger areathan the flank surface 18. In other words, the flank surface isrelatively narrow. For this reason, the flank surface may be ground in asimple and efficient way, whereby the narrowed plinth surface providesgreat freedom to make the flank surface and thereby the cutting edge,with the desired shape. A particular degree of freedom in this respectis accounted for below.

[0035] As has previously been pointed out, he insert has a marked axialangle (within the range of 10-15°) in the assembled state. By formingthe major edge portion 20 with an elliptic arc-shape with a certain archeight B, the insert may be brought to theoretically generate anentirely plane and smooth internal surface 9 on the walls 7 (see FIG. 3)which delimit a milled cell 6. During practical milling, however,deviations from the theoretic path of motion of the cutting edge, mayoccur. In order to avoid defects in the machined surface 9, e.g. in theform of convexly curved surface portions together with inwardly opening,crack generating flute formations, the major edge portion 20 may beformed with an arc height that is somewhat larger than the theoreticalarc height which generates an entirely plane surface 9. In case themajor edge portion 20 in this way is given an exaggerated arc height,the same will generate concavely curved part surfaces 9′ between whichthere are peaks 9″ directed outwards from the wall 7. In FIG. 3, saidconcavely curved part surfaces 9′ are shown on an exaggerated scale. Amachined surface having such, very slightly marked concavities ispreferable to machined surfaces having convexly curved part surfaces.Particularly within the aircraft industry, where supporting componentsin aluminium are manufactured by milling cavities or cells in solidworkpieces, partially concave surfaces of the type that is shown in FIG.3 are preferred.

[0036] As has previously been mentioned, the two cutting edges 19 formprimary cutting edges. Between said primary cutting edges along with theappurtenant chip surfaces 17, the border-like material portionsurrounding the valley surface 26 is countersunk. More precisely, twosecondary chip surfaces 34 in connection with two secondary cuttingedges 35 are countersunk in relation to the chip surfaces 17.

[0037] The individual primary cutting edge 19 is advantageously—thoughnot necessarily—sharply ground, more precisely by the chip surface 17 aswell as the flank surface 18 being ground. In this context, it should bepointed out that the individual chip surface 17 in its entirety is planeand inclined in two different directions in relation to the neutralplane N. Thus, in a first, substantially axial direction, the chipsurface 17 is inclined inasmuch as a front portion of the same adjacentto the minor edge portion 21 is situated at a greater distance from theneutral plane N than a rear portion of the same. Said axial angle ofinclination is in FIG. 7 designated P. The angle P may be within therange of 1-10°, suitably 3-50, and amounts in the example to 4°.

[0038] In a second substantially radial direction the chip surface 17 isinclined inasmuch as the distance between the neutral plane and the end(the point 24) which is situated closest the X-axis is smaller than thedistance between the neutral plane and other parts of the chip surface.This angle of inclination is designated a. The angle c in question maybe within the range of 2-15°, suitably 8-12°. In the example it amountsto 10°.

[0039] It should be pointed out that the primary chip surfaces 17 aswell as the secondary chip surfaces 34 internally transform into atransition surface 34 which extends obliquely downwards/inwards towardsthe valley surface 26. Said transition surface 36 may advantageously beconcavely curved. Irrespective of whether the primary cutting edges 19are sharp ground or not, the secondary cutting edges 35 may, if desired,remain unground. However, the secondary cutting edges may besingle-ground, more precisely by grinding only the flank surface 37. Thechip surfaces 34 may advantageously be of a concavely curved shape,which is achieved directly in connection with press moulding.

[0040] The clearance angle 106 of the flank surface 18 (see FIG. 10) mayadvantageously be within the range of 15-300, suitably 20-25°. In theexample, the clearance angle Ω amounts to 25°. Thus, if the angle aamounts to 100, the acute angle between the surfaces 17, 18 adjacent tothe major edge portion 19 becomes 55°. In other words, the insert has avery positive cutting geometry.

[0041] Reference is now made to FIGS. 9, 10 and 12, which closerillustrate the character of the connecting surfaces 4, 5. Each one ofthe two spaced-apart connecting surfaces 4 which are present on thebottom side of the insert comprises a plurality of long narrow, mutuallyparallel ridges 38, which are spaced-apart by grooves 39. The individualridge is of a cross-section-wise tapering shape. More precisely, theindividual ridge is delimited by an apex surface 40 (see FIG. 12) aswell as two flanks 41, which are inclined at an angle of 60° to eachother. In an analogous way, the connecting surface 5 forced in themilling head comprises a plurality of parallel ridges 42, which arespaced-apart by grooves 43 and which are delimited by apex surfaces 44as well as flanks 45. In this context, it should be pointed out that thematerial in the insert 3 (which may consist of conventional cementedcarbide or the like) usually is considerably harder than the material inthe milling head (which may consist of steel, aluminium or the like).Here it should be mentioned that the insert also may be mounted on anintermediate shim having ridged connecting surfaces.

[0042] In the preferred embodiment of the connecting surfaces 4 and 5which is shown in FIG. 12, surface portions 46 having a convexly curvedshape are forme=on the flanks 41 of the ridges 38, which portionsguarantee well-defined contact points between the co-operating flanks ofthe respective ridges. In practise, said curved surface portions areextremely small. Thus, the arc height may amount to 0.5-5% of the lengthof a conceived chord between the limiting edges of the surface, wherebythe arc height expressed in absolute measurements may vary within therange of 0.0015-0.015 mm.

[0043] The preferred embodiment of the connecting surfaces 4, 5according to FIG. 12 is described in detail in the applicant'ssimultaneously filed Swedish patent application with the denomination“Verktyg samt skärkropp för sp

navskiljande bearbetning”. The content of this parallel patentapplication is hereby incorporated in the present application byreference.

[0044] Below, a concretized embodiment follows of the milling insertaccording to the invention in respect of dimension and angle statements.The insert may be of a length L (see FIG. 8) of 30 mm and a width W of16 mm, whereby the thickness T (see FIG. 10) amounts to 5 mm. The radiusR of the individual minor edge portion 21 at each one of the acutecorners of the insert is 5 mm. In doing so, the angle of inclination τbetween the individual secondary cutting edge 35 (which is straight) andthe Y-axis amounts to 28°. As mentioned above the angle α amounts to1.8°, the angle β to 4°, the angle δ to 10°, and the angle Ω to 25°. Thecentral, rotationally symmetrical shaped hole 25 tapers in the directionfrom the topside of the insert towards the bottom side. The largestdiameter D₁ of the hole amounts to 8 mm, while the smallest diameter D₂amounts to 5.7 mm. On tightening, a conical surface of the screw head 28is pressed against a convexly curved abutment surface 47 in the hole.

[0045] The Advantages of the Milling Insert According to the Invention

[0046] A substantial advantage of the insert according to the invention,is that the major edge portion of the active cutting edge thanks to thearc-shape thereof generates the desired surface (entirely plane orhaving concave part surfaces according to FIG. 3) at the same time asthe connecting surface or surfaces formed by ridges, in cooperation witha corresponding connecting surface of the milling head ensure an exact,well-defined location of the major edge portion in relation to thecentre line or rotation axis of the milling head. Thanks to theinelastic fixing of the insert which is attained via the connectingsurfaces 4, 5, the radial distances between the two opposite end points22, 23 of the major edge portion may not only be initially fixed veryexactly, but also be maintained constant during operation. Anothersubstantial advantage is that the markedly positive geometry, which isattained not only by the own cutting geometry of the insert but also bythe positive tool geometry (i.e. by the fact that the insert seat isangled axially as well as radially), implies that the chip-removingcapacity of the insert and the milling tool becomes very high, at thesame time as the desired structure of the machined surface is obtaineddirectly in connection with the milling. In other words, noafter-treatment of the milled surface is required. In this context, itshould be pointed out that the rounded minor edge portion of the activecutting edge in a conventional way generates a plane surface 10 on thefloor in the milled cell. Another substantial advantage is that theinsert may be provided easily with extremely sharp cutting edges, moreprecisely by simple grinding the chip and clearance surfaces attached tothe respective cutting edges. Hereby, the narrowed plinth surfaces underthe flank surfaces guarantee that the grinding operation may be carriedout fast, easily and with minimal energy consumption. It may also bementioned that the co-operating connecting surfaces engaging in eachother by edge action secure the insert in the insert seat of the millinghead in a very reliable way. In other words, the insert will remain inplace even if it is exerted to such centrifugal forces that may arisewhen the milling head is driven at an extremely high rotational speed,e.g. 10 000 r/min or more.

[0047] Feasible Modifications of the Invention

[0048] The invention is not solely limited to the embodiment describedabove and shown in the drawings. Thus, it is, for instance, feasible togrind or in another way form strengthening chamfering surfaces, inconnection with the two cutting edges. Furthermore, it is feasible topolish the chip surfaces adjacent to the cu ting edges. The invention isalso applicable to other inserts than those that are provided with holesfor fixing screws. For instance, the insert may be fixed by means ofclamps. In such cases, a heightened material portion on the valleysurface of the insert may be formed with a countersink or a seat, theshape of which has been adapted to the shape of the clamp nose. Althoughridges having concavely curved flank surface are preferred in theconnecting surface or surfaces of the insert, it is also feasible toform the ridges with plane flanks of a conventional type. Furthermore,the insert may be of a rhombic shape instead of rhomboid. Although theinsert according to the invention above has been described above all formachining of such soft materials as aluminium, the same may, along withthe appurtenant milling cutter, also be used for machining of other,suitably metallic materials, such as steel, titan and the like.

1. Indexable milling insert having a polygonal basic shape and agenerally positive cutting geometry, including a top side (11), a bottomside (12) and four peripheral sides (13, 14), a first pair of mutuallyopposite sides (13) which meet a second pair of mutually opposite sides(14) in two acute angled corners (15) as well as two obtuse angledcorners (16), and also two cutting edges (19) for the cutting ofmaterial formed between chip surfaces (17) on the top side of the insertand flank surfaces (18) along the peripheral sides , which edgesindividually comprise on the one hand a major edge portion (20), and, onthe other hand, a rounded, minor edge portion (21), formed in connectionwith an acute angled corner (15), which minor edge portion extends inextension of the major edge portion (20) and the individual major edgeportion (20) is at least partly arched, characterized in that in saidbottom side (12) at least one connecting surface (4) is formed of thetype that comprises a plurality of parallel ridges (38) spaced-apart bygrooves (39) having a cross-section-wise tapering shape and which isintended to cooperate with an analogous connecting surface (5) inconnection with a chip channel (2) in an insert-carrying milling head(1), and that the two cutting edges (19) are formed on border-like,peripheral material portions in relation to which a valley-like surface(26) on the top side of the insert is countersunk and centrally on thevalley surface (26) is formed a raised material portion (27) forreceiving a means of clamping (29) for fixing the cutting insert in theinsert seat.
 2. Milling insert according to claim 1, characterized insaid raised material portion consists of a collar (27) formed around thehole (25), in which a screw head (28) which is part of a fixing elementin the form of a screw (29) may be housed in a countersunk and protectedstate.
 3. Milling insert according to claim 2, characterized in that thecollar (27) has an oval shape and extends diagonally essentially in thedirection between the obtuse angled corners (16) of the insert. 4.Milling insert according to claims 2 or 3, characterized in that twoconnecting surfaces (4) that are provided with ridges are formed on bothsides of a central hole (25) in the insert, more precisely spaced-apartfrom each other as well as from the hole.
 5. Milling insert accordingany of the preceding claims, characterized in that the arc height (B) ofsaid major edge portion (20) amounts to 0.05-1% of the length (L_(k)) ofa conceived chord (K), which extends between the opposite ends (22, 23)of the major edge portion.
 6. Milling insert according to any of thepreceding claims, whereby the shape of the insert is symmetrical inrelation to two co-ordinate axes in a conceived co-ordinate system, viz.a first co-ordinate axis or X-axis, which extends parallel to the ridges(38) and the grooves (39) in said connecting surface (4) and is situatedhalfway between the two major edge portions, and a second co-ordinateaxis or Y-axis, which extends perpendicularly towards the X-axis and issituated halfway between the acute corners (15) of the insert,characterized in that the individual major edge portion (20) extendsobliquely in relation to the X-axis inasmuch as a front end (22) of thesame, positioned nearest the minor edge portion (21), is situated at agreater distance from the X-axis than a rear end (23) of the same. 7.Milling insert according to claim 6, characterized in that the angle ofinclination (α) between the major edge portion (20) and the X-axis iswithin the range of 0.5-30, suitably 1.5-20.
 8. Milling insert accordingto any one of the preceding claims, characterized in that the same has arhomboidal basic shape, whereby the arched major edge portions (20)extend along the long sides (13) of the insert.
 9. Milling insertaccording to any one of the preceding claims, characterized in that theindividual cutting edge (19) is ground sharp by grinding both theconnecting chip and flank surfaces (17, 18).
 10. Milling insertaccording to any one of the preceding claims characterized in that theflank surface (18) extends from the cutting edge (19) to a shoulder (32)which delimits a narrowed plinth surface (33).
 11. Milling insertaccording to claim 10, characterized in that the. area the of the flanksurface (18) is smaller than the area of the plinth surface (33). 12.Milling insert according to any one of the preceding claims,characterized in that the arc height (B) of said major edge portion (20)is greater than a conceived nominal arc height which is required inorder that the major edge portion shall generate a plane surface,whereby the enlarged arc height ensures that the major edge portiongenerates a concavely curved surface.